In the manufacture of paper webs, such as tissues, towels, wipers, and fine paper, it is necessary to carefully control the cross-machine direction (CD) formation of the sheet in order to maintain the desired uniformity and properties of the final product. For products consisting of a single layer of fibers, the control of the CD profile has been proven in prior art. However, for multi-layered products in which a layered headbox is used to simultaneously deposit multiple layers of different fibers into the forming zone, merely controlling the overall CD basis weight profile through control of a single layer or equal control of all layers is not adequate, since even when the overall CD profile is uniform, there may be off-setting irregularities within each of the layers that have a significant impact on the final sheet properties. For tissue papers, these properties include, but are not limited to, tensile strength, layer purity, opacity and hand feel.
Therefore, in order to ensure that good sheet properties are consistently obtained when making multi-layered sheets, there is a need for a method of independently controlling the CD basis weight profile of individual layers of fibers within the sheet.
It has now been discovered that the stock approach piping to a multi-layered headbox can be modified to include a dilution water header that independently.supplies dilution water, such as whitewater, to multiple CD segments or zones of the approach to the headbox or the tube bank of the headbox nozzle which supplies stock to each layer. The CD basis weight profile of the final sheet is monitored and, in response to the measured CD profile, dilution water flow is increased or decreased to those tube bank segments that need their consistency altered. By selectively controlling the basis weight of distinct segments of the CD profile for each layer, a more uniform sheet can be produced.
Hence in one aspect, the invention resides in a method of controlling the CD basis weight profile of a layered paper web comprising:
(a) forming a layered paper web by using a layered headbox to deposit into a forming zone two or more aqueous suspensions of fibers, each of which corresponds to a different layering channel within the layered headbox;
(b) providing each layering channel with an aqueous suspension of fibers through a tube bank comprising a multiplicity of tubes arranged in the cross-machine direction of the headbox;
(c) measuring the cross-machine direction basis weight profile of one or more of the layers in the web or the dried sheet; and
(d) in response to the measurement of the cross-machine direction basis weight profile, providing dilution water to selected tube bank tubes to locally adjust the basis weight of that layer. As used herein, xe2x80x9cfibersxe2x80x9d can include cellulosic or synthetic fibers.
More specifically, the invention resides in a method of controlling the CD basis weight profile of a layered paper web comprising:
(a) forming a layered paper web by using a layered headbox to deposit into a forming zone two or more aqueous suspensions of fibers, each of which corresponds to a different layering channel within the layered headbox;
(b) providing each layering channel with an aqueous suspension of fibers through a tube bank comprising a multiplicity of tubes arranged in the cross-machine direction of the headbox;
(c) measuring the cross-machine direction overall basis weight profile of the web or dried sheet;
(d) measuring the cross-machine direction basis weight profile of one or more of the layers in the web or the dried sheet; and
(e) in response to the measurement of the overall cross-machine direction basis weight profile and the cross-machine direction basis weight profile of said one or more layers, providing dilution water to selected tube bank tubes to locally adjust the basis weight of that layer.
In another aspect, the invention resides in a method of controlling the CD basis weight profile of a layered paper web comprising:
(a) forming a layered paper web by using a layered headbox to deposit into a forming zone two or more aqueous suspensions of fibers, each of which corresponds to a different layering channel within the layered headbox;
(b) providing each layering channel with an aqueous suspension of fibers through a tube bank comprising a multiplicity of tubes arranged in the cross-machine direction of the headbox;
(c) measuring the cross-machine direction profile of at least one basis weight-related property, such as tensile strength, opacity, or layer purity, of one or more of the layers in the web or the dried sheet; and
(d) in response to the measurement of the basis weight-related property profile, providing dilution water to selected tube bank tubes to locally adjust the basis weight of one or more layers.
In another aspect, the invention resides in a method of controlling the CD basis weight profile of a layered paper web comprising:
(a) forming a layered paper web by using a layered headbox to deposit into a forming zone two or more aqueous suspensions of fibers, each of which corresponds to a different layering channel within the layered headbox;
(b) providing each layering channel with an aqueous suspension of fibers through a tube bank comprising a multiplicity of tubes arranged in the cross-machine direction of the headbox;
(c) measuring the cross-machine direction basis weight profile of one or more of the layers in the web or the dried sheet;
(d) measuring the cross-machine direction overall basis weight profile of web or dried sheet;
(e) measuring the cross-machine direction profile of a basis weight-related sheet property, such as tensile strength, opacity, or layer purity, of one or more of the layers in the web or the dried sheet; and
(f) in response the measurement of the cross-machine direction overall basis weight profile and the cross-machine direction layer basis weight profile or a basis weight-related sheet property, providing or eliminating dilution water to selected tube bank tubes to locally adjust the basis weight of that layer.
The number or frequency of the stock tubes which are controllably fed by dilution tubes in the cross-machine direction of the headbox can vary with the application. Depending upon the frequency and arrangement of the stock tubes, it may or may not be desirable to have one dilution tube for every stock tube. The purpose is to be able to adequately modify the local basis weight of the layer in question sufficiently to reach the desired target basis weight. This will depend on the degree of variability that is acceptable for the given product, the speed of the machine, the flow rates within the tubes, the number of tubes, etc. In general, however, from about 1 to about 24 dilution tubes per foot of cross-machine deckle are believed to be suitable.
The means for determining if a dilution adjustment is needed can be any means that directly or indirectly measures the profile of the cross-machine direction basis weight, or a related property, for one or more layers. The measurement can be made on the wet web or the dry sheet. It can also be made on-line or off-line. While on-line measurements can be preferred because of their fast responsiveness, off-line measurements are also useful because many of the cross-machine irregularities remain relatively constant over long periods of time if the paper making process is adequately stabilized or controlled. Therefore, the fast response of on-line measurements is not always necessary. Suitable measurement techniques include, without limitation, the following:
(a) Infrared spectroscopy. This technique can detect different absorption wavelengths for different fiber types. Infrared spectroscopy can also be used to detect the presence of chemical additives or fiber structural changes due to mechanical treatment of the pulp, or other pulp characteristics. For instance, when using near infrared spectroscopy, analysis of the 2nd derivative of the absorption spectra may be used to differentiate hardwood from softwood pulps. Also, infrared spectral differences may be employed using specular reflectance. Traditional off-line absorption spectroscopy in the 800-900 cmxe2x88x921 regions can be used as well. Both methods are readily available off-line.
(b) Addition of a chemical trace. Commonly used chemical additives, such as wet strength additives or dry strength additives, can be detected using known analytical techniques. Also, a new chemical additive can be added for the sole purpose of measuring its presence, such as adding an optical brightener, which can be detected using ultraviolet technology.
(c) Measuring opacity. An on-line or off-line technique where the opacity of the paper is measured using a suitable sensor. The opacity of the sheet can be measured in several cross machine sections corresponding to the dilution zones. A low opacity would prompt removal of dilution water to increase the basis weight of a selective layer.
(d) Image analysis. This technique can be used to detect different physical fiber characteristics, which could be natural characteristics or characteristics imparted by mechanical pretreatment. The identification of certain fiber types would indicate the ratio of the fibers present from given layers of the sheet. This information in combination with basis weight data would be used to manipulate the dilution in a specific layer to control weight while maintaining desired properties. This could be an on-line or off-line measurement.
(e) Image spectroscopy. This technique could include the manipulation of vibrational spectroscopy with a camera to measure fiber density changes. Different fiber types may be denser than others. This information in combination with basis weight data would be used to manipulate the dilution in a specific layer to control weight while maintaining desired properties. This could be an on-line or off-line measurement.
(f) Measuring tensile strength. This would be an off-line test. A full cross machine direction sample would be cut into smaller widths depending on the frequency of the dilution addition to the headbox. The tensile strength of each section would be measured and evaluated with the basis weight of the same segments to determine where the basis weight discrepancy is present. For example, if there is a strength deficiency in the same segment, there could be a deficiency of fiber in the strength bearing layer and the dilution profile would be changed appropriately.
(g) Measuring layer purity. This is also an off-line test. Tape pulls in the z-direction of the cross machine direction paper segments would be tested. The count of fibers in each layer is made using the structure of the fiber as the distinguishing factor. This information in combination with basis weight data would be used to manipulate the dilution in a specific layer to control weight while maintaining desired properties.
There may be other means of measuring these and other paper web qualities that may prove to be useful in enhancing the quality of tissue products.